1. Field of the Invention
The invention relates to permeable membranes for the separation of fluid mixtures. More particularly, it relates to composite membranes having enhanced separation/permeability characteristics and methods for their preparation.
2. Description of the Prior Art
Permeable membranes capable of selectively permeating one component of a fluid mixture, either gas or liquid, are considered in the art as a convenient, potentially highly advantageous means for achieving desirable fluid separations. For practical commercial operations, permeable membranes must be capable of achieving an acceptable level of selectivity of separation of the gases or liquids contained in a feed stream while, at the same time, achieving a desirably high productivity of fluid separation.
Various types of permeable membranes have been proposed in the art for the carrying out of a variety of fluid separation operations. Such membranes can generally be classified as being of the (1) isotropic, (2) asymmetric or (3) composite type. The so-called isotropic and asymmetric type membranes are comprised essentially of a single permeable membrane material capable of selectively separating desired components of a fluid mixture. Isotropic membranes have the same density throughout the thickness thereof. Such membranes generally have the disadvantage of low permeability, i.e. low permeate flux, due to the relatively high membrane thickness necessarily associated therewith. Asymmetric membranes are distinguished by the existence of two distinct morphological regions within the membrane structure. One such region comprises a thin, dense semipermeable skin capable of selectively permeating one component of a fluid mixture. The other region comprises a less dense, porous, non-selective support region that serves to preclude the collapse of the thin skin region of the membrane under pressure.
Composite membranes generally comprise a thin layer or coating of a suitable permeable membrane material superimposed on a porous substrate. The separation layer, which determines the separation characteristics of the composite structure, is advantageously very thin so as to provide the desirably high permeability referred to above. The substrate only serves to provide a support for the thin membrane layer positioned thereon.
As the advantages of permeable membranes have become increasingly appreciated in the art, the performance requirements of such membranes have likewise increased. Thus, the art is moving in the direction of very thin membranes having desirable permeability characteristics without sacrifice of the separation, or selectivity, characteristics of the hollow fiber or other permeable membrane structure. It is thus increasingly desired that more advantageous combinations of permeability and selectivity be achieved with respect to a variety of fluid separations of commercial interest. As indicated above, isotropic-type membranes are not generally suitable for the achieving of such requirements. Asymmetric membranes, on the other hand, can be developed for such practical fluid separation applications, but do not possess an inherent flexibility enabling them to be readily optimized for particular fluid separation applications. While the thin dense, semipermeable layer of a particular asymmetric membrane material can be made thinner for increased permeability, the selectivity characteristics of said material, unless modified by particular treatment techniques, may be no more than adequate with respect to the separation of the components of a fluid mixture being treated in a particular application.
The thin skin of such asymmetric membranes, which are described in the Loeb patent, U.S. Pat. No. 3,133,132, is frequently found not to be sufficiently perfect for gas separation operations, but to contain various imperfections or defects. Such defects, in the form of residual pores, minute pinholes and the like, comprise relatively large size openings through which a fluid mixture will preferentially flow. As a result, a significantly reduced amount of fluid separation will occur as a result of the presence of such defects in the membrane structure. In the case of asymmetric polysulfone hollow fibers, such defects result in the selectivity (as defined below) being only in the range of about 1-1.5 as contrasted to a selectivity of about 6.0 for polysulfone that is free of defects. In a proposed solution to this problem, Henis et al., U.S. Pat. No. 4,230,463, disclosed an asymmetric membrane coated with a material having a determined intrinsic separation factor that is less than that of the material of the separation membrane and exhibiting a separation factor significantly greater than the determined intrinsic separation factor of the coating material and greater than that of the uncoated separation membrane. Using this approach, silicone, having a selectivity of about 2, can be coated on polysulfone hollow fibers to increase the selectivity thereof from the 1-1.5 range indicated above to from 2 to 6, with such selectivity commonly approaching 6. The permeability (as defined below) of such silicone/polysulfone composites have generally been relatively low, i.e. about 0.2 ft..sup.3 (STP)/ft..sup.2. day . psi or less, leading to the desire for thinner membranes, i.e. thinner dense skins, particularly in light of the increasing requirements in the art for high flux operation. Thinner membranes lead, however, to an increase in the number of defects that require curing to achieve acceptable performance. While efforts to improve this approach continue, there remains a desire in the art for other approaches to provide a desirable combination of selectivity and permeability for practical commercial operation. For such reasons, composite membranes, utilizing membrane materials selected particularly for a desired gas or other fluid separation, offer the greatest opportunity, with respect to particular gas separations of commercial interest, for the achieving of desirable combinations of selectivity and permeability. The requirements for composite membranes are not only that the separation layer be very thin, but that the material of the separation layer be optimized for the desired fluid separation application. One such application of significant commercial interest is air separation, particularly wherein the membrane material selectively permeates oxygen for recovery as an oxygen-enriched permeate gas, with a nitrogen-enriched stream being withdrawn as non-permeate gas. There is a genuine need and desire in the art, therefore, to develop a composite-type membrane particularly suitable for air separation hydrogen-methane, hydrogen-nitrogen, carbon dioxide-methane separations, and other desirable gas separation operations.
Such composite membranes are also desired in fluid separation processes that involve a phase change of one or more components of the mixture to be separated. The feed and the permeate streams are thus alternately in the liquid and gaseous state in such processes, with gas being present on one side of the membrane. An example of such a process is pervaporation through membranes, which is particularly useful in the separation of liquids from their azeotrope solvent mixtures, and wherein liquid is present on the feed side of the membrane. Another such process is perstruction, wherein liquid is present on the permeate side of the membrane.
It is an object of the invention, therefore, to provide a composite membrane having an advantageous combination of selectivity and permeability for desired gas, pervaporation or perstruction separation operations.
It is a further object of the invention to provide a process for the preparation of composite membranes having such advantageous combination of selectivity and permeability for desired gas, pervaporation or perstruction separations.
It is another object of the invention to provide a composite membrane particularly suitable for air separation applications.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.